1. Let's look at some English sounds first. Put your fingers on your throat and say "b". Then say "p". When you say the second sound, your vocal chords stop vibrating, that's a voiceless sound. They might vibrate a bit, because we tend to add a bit of a vowel or other, but for the sound itself they don't. The IPA symbols for these two sounds are [b] and [pʰ], respectively. To put it in exact terms, [b] is a voiced bilabial plosive, i.e. the vocal chords vibrate, you produce it with both of your lips, and it's a stop sound. [pʰ], on the other hand, is a voiceless aspirated bilabial plosive. The voiceless part means that your vocal chords don't vibrate, and the aspirated part means that you add a little puff of breath to the end of it. Aspiration is marked in the IPA with a superscript h.
2. Now let's do the same for Gaelic. In Gaelic, when you say "p", it's conveniently just the same, a pʰ. Not for "b" though; it looks like a voiced sound, but it isn't! Rather, it's a devoiced sound: [b̊], with the little circle on top signalling devoicing. Don't ask me what the difference between a devoiced and a voiceless sound is, just treat it as voiceless for practical purposes. So, what you do is say "p" and remove the little puff of breath at the end of the sound.
3. Do the same for "t" - "d" and "k" - "g", which are [tʰ] - [d̪̊] and [kʰ] - [g̊] in Gaelic. There are actually two more of these pairs, but let's leave them aside for now. One little complication: Notice the little bracket in the [d̪̊] sign. This signals a dental sound, which means that you put the tip of the tongue against your teeth. So, it shifts a bit forward. Don't be frustrated if you can't do that at first, you'll be perfectly well understood. What counts is the aspirated - not aspirated distinction.