Slippage is the idea that, if you buy $1 of bitcoin then you are probably going to lift the best/lowest offer. Whereas if you were to buy $100m of bitcoin, the best offer is not going to be enough, you have to go deeper into the order book and start lifting more and more offers at less and less favorable prices.
So ultimately when you get that order executed your blended average price of that execution is much worse than if you were to just trade $1. So in theory the larger the size that you are trading the worse the average blended price you are going to get on that block.
Since people trading these large blocks face this issue, OTC desks are there to service this problem and hopefully reduce costs/slippage for the trader or investor.