Hello, Vido, and welcome!
To play with a bid, give the Allied team some money (“Industrial Production Credits,” or IPCs) before the game starts. The Allies can choose which player or players will get the money. Those players can then spend the money to buy units that get placed onto the game board before the game starts. Or, the players can keep the money and spend it on a later turn. It is their choice.
For example, suppose the Allies get a bid of 10 IPCs. The Allies decide to split the money up so that Russia gets 6 IPCs and Britain gets 4 IPCs. Russia could decide to place one submarine on the board (submarines cost 6 IPCs), or Russia could place two infantry on the board (infantry cost 3 IPCs each), or Russia could simply save the money and use it later. Britain could decide to put an extra artillery on the board (artillery costs 4 IPCs), or to put an infantry on the board and save 1 IPC, or to simply save all of the money.
There are sometimes some additional rules about where you can place units (often only where you already have at least one unit) and how much the bid should be (often based on a competitive auction), but those are the basics.
The larger the bid, the more generously you are helping the Allies. For example, a bid of 30 IPCs is very large and suggests that either you think the game is biased in favor of the Axis or that the Axis players are more skillful than the Allied players and so the Allied players will need a lot of help. A bid of 5 IPCs is very small and suggests that you think the game is pretty close to balanced even without a bid and so the Allies do not need much help.