It could also mean because (something) is there. 서 is what means because. This sentence is not complete, though. Example: 네가 내 곁에 있어서 나는 해복해요. Because you're here, I'm happy. (I'm happy because you're here.)

It's the possessive verb like "have" or "has". The last syllable 서 indicates that there is another verb in the sentence after it. Like say your at a restaurant, you could ask them "cola 있어요?" Then they might respond "네, cola 있어요."

it`s not easy to explain. i think you need to show us the all sentence. that`s better. cant use 있어 , 서 separately. if you say only " 서 " thats not same with "because".