Pigg has a few different uses, but the most common one is as an adjective, meaning 'alert/sprightly/wide awake/cheery/lively'. Basically, it refers to someone with plenty of energy, and is roughly the opposite of being tired and grumpy.
Sometimes it simply means 'awake and alert', for example man måste hålla sig pigg när man kör bil (you need to stay alert when you're driving), and sometimes it implies liveliness and possibly more noise or movement than average as well, for example if you describe en pigg publik (a lively audience) or pigga barn (lively children).
Understanding this means that the Swedish name for Mickey Mouse, Musse Pigg, suddenly makes a lot more sense. And Harry Potter fans may appreciate that the fictional Weasley family's owl, Pigwidgeon, is renamed Piggy in the Swedish translation, which is especially fitting given the bird's hyperactive behaviour.
You could describe this fellow as a pigg pig (except that 'pig' is gris in Swedish) via GIPHY
You'll often hear pigg used as part of a set phrase which usually clarifies the exact meaning, for example pigg och glad (wide-awake and happy), pigg och alert (wide-awake and alert), pigg och kry (fit and healthy). Two common idioms in Swedish are to say someone is pigg som en lärka (sprightly as a lark) or pigg som en mört (as alert as a common roach).
And you can follow pigg with the preposition på to say 'keen on' or 'enthusiastic about' something, for example han är pigg på att jobba (he's keen to work/he's enthusiastic about his work/he hasn't got tired of working).
Pigg also exists as a noun meaning 'spike', and it's therefore likely that the two meanings are related. For example, porcupine is translated as piggsvin in Swedish, literally 'spike-pig'.
Oj, du ser pigg och glad ut
Wow, you look wide awake and happy
Nej tack, jag är inte så pigg på det
No thanks, I'm not too keen on that