The word sneak has many shades of meaning, but all involve doing something in a secretive or stealthy way. If you sneak home after midnight, that means you’re quietly creeping into the house so no one will hear you.
In addition to moving furtively, sneak can also mean doing something secretly or slipping something in. You might sneak a cookie when no one is looking, or you might sneak your brother into a concert. You could even sneak a glance at that cute guy in your math class — you do it when you think no one will notice. Sneak has a noun form that describes a furtive person — someone who does a lot of sneaking.