The "five love languages" are based on Gary Chapman's 1992 book, The Five Love Languages: How to Express Heartfelt Commitment to Your Mate. Essentially, the concept is that there are five main ways people give and receive love: through acts of service, receiving gifts, quality time, words of affirmation, and physical touch.
Just in case you needed a refresher, here are some examples of each love language:
Acts of service — making your SO coffee in the morning or running an errand so that they don't have to. It can also be something bigger like breakfast in bed.
Giving/receiving gifts — this can of course apply to big gifts, but smaller gifts like random flowers or giving them their favorite candy bar also count.
Quality time — being together without distractions and giving each other your undivided attention can entail activities like going on a hike, having dinner, or just making time to talk about your day.
Words of affirmation — compliments, duh. But this also includes showing verbal appreciation, encouragement, and saying "I love you."
Physical touch — cuddling, kissing, holding hands, giving each other massages, hugging, and having sex are all example of this.