The first player to bid starts with a high number (for Allies) then the next player bids a lower number, and this goes down auction style until the other player says “yours”.
So then you spend the bid amount on units that you place on the board prior to the first turn. In TripleA you go to “Game Options” and give the nation you want the extra bid number…then when the game starts you’ll have the opportunity to buy units with the bid and place them before the 1st turn.
The best bid either strengthens the UK’s defenses against G1 attacks or strengthens UK attacks on UK1. So a fighter to Scotland is great because it can be scrambled to sz 111 or sz 109 on G1 AND it can be used for any number of attacks on UK1. A submarine to sz98 is extremely common because it’s a good idea (if it’s possible) for UK to attack the Italian units in sz97 on turn 1.
Other potential bids include extra Chinese units, extra Russian fighters (these can be flown into China on R1), extra British units in Africa (to attack Ethiopia or Tobruk on UK1, or just to strengthen Egypt’s defenses), and extra UK subs (110, 109, 106, 91 are all good places to put an extra sub).
If you got a 20 bid, imho a good bid would be fighter to Scotland, ss to 98, and an inf/art/mech somewhere else (sometimes it’s allowed to place an extra ANZAC inf in New Guinea which is good because it allows Allies to seize Dutch NG on turn 1…but others will place the extra unit on Alexandria or Egypt or Sudan or on the United Kingdom).