Learn German: Weht der Wind stark, haltet euch gut fest! - If the wind blows strongly, hold on tight!

This is a conditional clause without wenn (if). In German, you can drop wenn to create an 'if' condition, but when you do, the conjugated verb must move to the very first position. So Weht der Wind stark means the exact same thing as Wenn der Wind stark weht.

Why is the verb haltet placed immediately after the comma?

The entire first clause (Weht der Wind stark) counts as Position 1 for the whole sentence. Since German requires the main verb in Position 2, the verb haltet must come right after the comma. It also happens to be a command, which normally starts with the verb anyway.

Why do we use euch in this sentence?

The verb sich festhalten (to hold on) is reflexive in German, so you literally command the group to 'hold yourselves tight'. Since the command is given to a group of people informally, we use the plural reflexive pronoun euch (yourselves).

Where is the subject in the second part of the sentence (haltet euch gut fest)?

There is no subject written out because it is an imperative (a command). When giving a command to a group of people you know informally, you use the ihr verb conjugation (haltet) but leave the word ihr out.