When programming music using MIDI in a sequencer we can align everything perfectly, beat by beat bar by bar. We can make sure all our notes are evenly played and exactly on the beat. Even with this, our music can sometimes sound "off". In it's perfection, it looses something, because in reality nothing is perfect. Every flaw can give character and identity that makes that sound unique. This is known as humanising. If a musician was to play the same part twice, they would both be slightly different
There are two things to consider when humanising midi notes, timing and tone. In software these can be manipulated through quantising and velocity settings.
Quantising - This more commonly is used to make sure parts played are in time. However, most software also allows you to set how tightly the notes are quantised and how this varies note to note.
Velocity - This is how hard the midi notes are played. The result effects the tone of the sound, high velocity could result in a louder, more harsh tone. Lower velocities can result in a softer, lighter tone.
When playing a piano for example, not every finger will play each note the same way, nor will they play at the exact same time. Taking the time to adjust these settings in your DAW will help your music feel more natural and can have interesting results when sounds are layered.
See the video below to find out how I do this in Propellerhead Reason.