Flat is a betting strategy. It implies a very prudent approach, when small amounts of money are used in betting, often no more than 15% of the player's entire bank.
Free bet is a bonus given by a bookmaker for registration. This implies betting free of charge. To withdraw it, you need to meet certain conditions. Often this implies a specified number of wagers made.
Friendly is a friendly match. Analysts advise against betting on it. The point is that teams are experimenting with playbooks and lineups, so winning is not crucial to them. Friendlies take place at the beginning of the season, at pre-tournaments.