Loose, lose, looser, and loser often cause problems among both native and non-native speakers. The reason is quite obvious: their spelling and pronunciation are rather similar. Just by counting the number of o’s, we can guess that lose and loser are related, as are loose and looser. However, the two word pairs have very different meanings.
The meanings of the verb ‘to lose‘ include the act of no longer having something or someone. For example,
to lose a friend,
to lose weight.
Another meaning is that of being defeated, as in
to lose a game.
The simple past is lost (we lost the game), which is also the corresponding adjective (to be lost). Related nouns are loss and, more importantly, loser. It refers to someone or something who/which loses something, such as a battle, game, or election. You can find more meanings here.
The pronunciation of lose is [luːz], listen here. Loser is pronounced as [luːzə(r)], listen here, with the optional r referring in particular to the American pronunciation. As you will learn in a minute, the pronunciation of ‘se’ as z is essential to avoid confusion.
The word looser is an adjective, namely the comparative of loose (the superlative is loosest). Loose is the opposite of tight. For example, you may say
If you lose weight, your pants will be looser.
Because looser does not exist as a noun, it makes no sense at all to call someone a looser! You could say that the meaning of your words would be lost on whoever you are talking to. I suspect that the reason why loser and looser are so often mixed up in written English is that both are legitimate English words that your spell checker will not complain about (unless it also checks for grammatical mistakes).
If you are not a native speaker, pronunciation is also an issue. The correct pronunciation of loose is [luːs], listen here. Looser is pronounced as [luːsə(r)]. Comparing these phonetic spellings to those for lose and loser above, you can see that they differ by how ‘se’ is pronounced: z for lose(r) but s for loose(r). This difference in pronunciation, which may appear rather subtle to non-native speakers, makes it possible to distinguish these words.
PS: The confusion surrounding loose, looser, lose, and loser is perfectly illustrated by the fact that my search for a suitable stock photo using the term ‘looser‘ mostly returned pictures related to losing money.