Idioms and colloquial expressions make your Spanish sound natural and fluent. These are phrases whose meanings can't be guessed word by word.

Everyday Expressions

• No hay mal que por bien no venga. — Every cloud has a silver lining.
• Más vale tarde que nunca. — Better late than never.
• En boca cerrada no entran moscas. — Silence is golden. (lit. Flies don't enter a closed mouth)

Common Colloquial Phrases

• ¡Qué padre! (Mexico) / ¡Qué guay! (Spain) — How cool!
• ¡Qué lástima! — What a shame!
• ¡Qué rollo! — What a bore! / What a hassle!
• Estar en las nubes. — To have your head in the clouds.
• Costar un ojo de la cara. — To cost an arm and a leg.
• Tomar el pelo. — To pull someone's leg / to tease.

Filler Phrases

• O sea... — I mean... / That is...
• Es que... — The thing is... / It's just that...
• A ver... — Let's see...

Reference:

Wikipedia: Spanish Language

https://en.wikipedia.org/wiki/Spanish_language