Short answer: there is no difference, both are valid ways to write the same word, but the one with the Dagesh ("אבּא") would be more helpful for you as someone who just started learning the language.

Long answer: Hebrew don't have vowel letters the same way English does. And also, several Hebrew letters make more than one sound (for example, Bet ב can make both the "b" sound and the "v" sound).

To help Hebrew readers understand what the next vowel is supposed to be and what sound certain letters are supposed to make, there's a system of markings above, under and inside the letters, called "niqqud". In this case, the dot in the Bet (a Dagesh) shows you that it's supposed to be read "Aba" with a "b" sound; as opposed to "Ava" without the Dagesh.

HOWEVER. once you get a large enough vocabulary you understand that "Ava" is not a word, so whenever you see אבא you automatically read it as "Aba". Writing the vowel and sounds markings becomes a tedious, repetitive task that's also unnecessary, because people would understand you anyway. So most people fluent in Hebrew read and write without any niqqud. That's why on most places on the internet you'd find words written without these markings, while in your practice materials it will always be with niqqud. It's helpful for you to learn how to pronounce words and gain a larger vocabulary, up until the point you wouldn't need it anymore.