Simple meaning of Yoga is to get attached to God or to get one with the God. To attain this, there are many types of Yogas that are expounded in the Bhagwat Gita, like Sankhyayoga or the Yoga of Knowledge, Karmayoga or the Yoga of Selfless Action, Bhaktiyoga or the Yoga of Devotion etc. Among all these Yogas, Karmayoga is the one through which one can attain God by just devoting all his/her daily actions to God i.e. doing service of God by doing/devoting all life actions to Him.
Below are the verses from "Bhagwat Gita-Chapter Twelve- Verse - 6-7", in which God is asking his disciple Arjuna to do all his actions / activities for Him only.
'Ye tu srvani krmani
myi sanyasya mat-para
maam dhyayant upasate'
bhavami na-chirat parth
mayy-avesita-chetsam' ( Bhagwat Gita: Chapter XII verse 6-7 )
'Those people, who worship Me by surrendering all their activities unto Me, are always devoted to Me, always meditate upon Me, Having always fixing their minds upon Me, O Parth - to them, I swiftly rescue from the ocean of birth and death.'
Through above verse 6 & 7, God is telling 'Parth' (another name of Arjuna, his disciple), that those people who work for God only & offer all their activities & work to Him only, to such a Soul He shows great mercy, always protects him & finally rescues him during & after death.
Now, how can we work for God only? This whole world is the creation of God only & knowingly, unknowingly, we all are working for the maintenance of this world only. Just introspect & see that, whatever, work or job (good one), you are doing in this world, it is helping in someway or the other in maintaining & sustaining this world. Just take it in that sense, that if all the people of the earth stop doing their daily work, duties, this whole world shall come to a standstill. In that case nobody shall grow food, vegetables, fruits & other things which sustain our life & hence we shall all die. Same is the case with other professions & each person is adding value to this world by doing his/her duty.
Now, though we do duty to sustain this world only but we don't realize that. As this world is a creation of God, so we should work for Him only. If we develop this sense that we are working selflessly to sustain this world created by God only, then also we shall develop an affinity towards God. In the above verses, God is making very clear that if a person offers all his work & activities to God, thinking this world to be a creation of God only, then also He shall be happy with that person & shall protect & shall deliver that person from the cycles of birth & death. By working for God, we shall attain peace & shall attain His love & devotion. So Let's all Work for God only.
Below is a verse from "Bhagwat Gita-Chapter Two Verse - 47",
"karmany evadhikaras te
ma phalesu kadachana
ma karma-phala-hetur bhur
ma te sango ’stv akarmani" (Bhagwat Gita: Chapter Two verse 47)
"Sri Krishna said: You have a right to perform your prescribed duty, but you are not entitled to the fruits of action. Never consider yourself the cause of the results of your activities, and never be attached to not doing your duty.
In the above verse God is asking his disciple Arjuna to perform his prescribed duty & not get attached to the fruits of its action. If one does all his/her actions without considering or thinking about the Fruits these actions shall produce & devotes that action & its fruit to the God only, then he/she is not bound by that action & that action shall not bear such fruits which shall be the cause of his/her next birth. Once a person is relieved of taking rebirths then that person can go to the abode of God & live there with Him & he becomes a liberated soul.
Art of doing actions for the sake of God only & doing all his/her actions without thinking or keeping in mind the fruit of that action is only what is called Karmayoga.