A contraction is a combination of two or more words written and pronounced as one word. For example, the contraction we're, which combines the pronoun we and the verb form are. Contractions are always written with an apostrophe that generally stands for the part of the contracted word that's been omitted.
Unfortunately, the pronunciation of contractions isn't always evident from the spelling. Some of them have more than one reading. For example, didn't is a contraction of did not and can be read [dɪ́ɖɪnt̚] or [dɪ́d̚ɴt̚] or [dɪ́t̚ɴt̚] or [dɪ́ʔɪnt̚] depending on the speaker. The best practice is to imitate the native speakers you communicate with and hear in blogs, movies and shows — assuming they speak English reasonably well.