phrase meaning - What exactly does "up to 3 months before" mean?

When it says:

You can apply for a visa up to 3 months before your date of travel to
the UK.

Source: gov.uk

Does it mean the latest I can apply for a visa is 3 months before the travel date, or the earliest I can apply for a visa is 3 months before the travel date?

Answer

There are a number of ways in which the statement may be analysed.

You can apply for a visa up to 3 months before your date of travel to the UK.

The ambiguity is in the interpretation of up to: it could mean until or it could mean a maximum of.

You can apply for a visa until 3 months before your date of travel to the UK.
You can apply for a visa a maximum of 3 months before your date of travel to the UK.

In this case, up to should almost certainly have the second meaning. There's no point in applying for a visa years before you intend to travel, because your circumstances could change in the meantime. So the authority seeks to limit the possibility of changes by forcing you to apply fairly close to the date of travel.