It means our thoughts, words and actions are aligned, it means communicating what we understand to be true.
We practice Satya when our hearts and minds are aligned, clean and unbiased.
When practiced alongside nonviolence (Ahimsa), creates a powerful force for goodness, and the other Yamas and Niyamas become much easier to practice.
Besides, if our thoughts, words and actions are harmful (not practicing Ahimsa), even if we are practicing Satya (all of them being aligned), we are not engaging in yoga. That’s why this Yama comes after Ahimsa.
These two Yamas combined means honest, appropriate, helpful, sensible, unselfish and peaceful communication, that takes into account the short and long term consequences, that aims for the greater good.
Satya means we will do what we say we will do. Practising it makes us responsible for our words, and reliable and trustworthy (or not).