Buy in

purchase a supply of (something); purchase something that provides membership or participation to a group (buy into a company, etc.)

Buying in has several meanings. In the securities market it refers to a process by which the buyer of securities, whose seller fails to deliver the securities contracted for, can 'buy in' the securities from a third party with the defaulting seller to make good. In poker it signifies the up-front payment required to participate in a given game or tournament. In the auction world it refers to unsold lots - the auctioneer will start the bidding on behalf of the seller up to the reserve amount, and if no-one outbids him, he is said to have bought-in the lot.
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Verb
1. stock up on to keep for future use or sale; "let's stock coffee as long as prices are low"
(synonym) stock
(hypernym) supply, provide, render, furnish
(hyponym) overstock
(classification) commerce, commercialism, mercantilism

To cover, offset or close out a short position. Related: evening up, liquidation.

The minimum required amount of chips that must be "bought" to become involved in a game (or tournament). For example, a $4-$8 fixed limit game might require a player to buy at least $40 worth of chips. This is typically far less than an average player would expect to play with for any amount of time, but large enough that the player can play a number of hands without buying more, so the game isn't slowed down by constant chip-buying.