Allocation refers to the amount a given person or fund is able to invest in a given company. For example, a company may be raising $1M, and they are allowing a certain investor or fund to invest $100K. That means they are giving this investor or fund the opportunity to invest $100K.
Note that getting allocation in a deal is often considered the most difficult part of venture capital. Investors or leads must convince founders that they should take money from them, versus a different investor or fund.