If East passes, the auction ends and 1♣ becomes the final contract. It is generally undesirable to let the auction die at such a low level, hence, East will usually bid or double in order to let the auction continue.
Bidding at the balancing seat differs slightly from bidding at the direct seat. In particular, preemptive jumps are not bid at the balancing seat. Generally, bidding at the balancing seat requires slightly less strength then at the direct seat, since the partner has been passed.
For example, holding the following hand, a takeout double is good at the balancing position but too weak at the direct position: