It depends on the context ,but it is used to express...
1) your opinion on something
2) your impression of something
3) your guess
Here are some examples of its usage.
It looks like rain - 비가 올 것 같다/같아(요).
= I think it's going to rain.
It seems to become spring - 봄이 온 것 같다/같아(요).
I think I need to go to see a doctor - 병원에 가야 할 것 같다/같아(요).
I think so - 그런 것 같다/같아(요).
I guess it will be interesting - 그거 재미있을것 같다/같아(요).