This is used to mean "as far as...", "until". For example:

• kilencig or kilenc óráig - until 9 (o'clock)
• Megyek a folyóig. - I'm going as far as the river
• Itt leszek két hétig - I will be here for two weeks (until two weeks).

Some more common uses are in the demonstrative pronouns and the question: Remember how most English wh- question words are m- in Hungarian.....

meddig? until when?

ez + -ig = eddig until this (time) = until now

az + -ig = addig until that (time) = until then

It's one of the few endings that has just one form, there are no options for vowel harmony. All nouns, no matter their harmonious character, take the ending -ig.

I have seen that some translators try to use the American style or showing a time-span, using the word "through". The phrase "I will be here for three days" may be translated as "Három napon keresztül itt leszek" which actually means "I will be here through three days." This isn't the same.