There are a number of things it could be, normally your notice the drag when you put the bike in gear and hold the leaver in, but the bike still wants to move forward. Clutch slip is when you release the clutch and go to accelerate from first but the clutch slips and does not engage forward motion for split second or two.
Both of these can be a result of the following -
- Incorrectly adjusted chain.
- Worn, un-lubricated chain.
- Incorrectly adjusted cable.
- Worn, un-lubricated cable.
- Worn 'worm-drive'
- Un-greased 'worm-drive'
- Incorrect oil.
- Worn clutch basket/plates.
So start with checking your chain adjustment, it can be other things, but best look at the simple stuff first.