This is a concept with which some folks have initial problems. In one sense, the word means to play well, to "cook", but here we mean a rhythmic shift of a more definite nature. Suppose we take a group of 4 eighth notes, shown here in box notation:

If o is a conga open tone, and x is a slap, the rhythm, played straight would sound like this:

conga pattern, 50% swing (none)

The second note, for example, is 50% of the way to the 3rd one, exactly half way

To swing this rhythm, you would delay the second and fourth notes a bit, sometimes quite a bit; now we'll swing the same rhythm. Listen to the difference; play one, then the other. This leaning over of the notes is what is commonly meant by swing.

conga pattern, 59% swing

In this example, the displaced notes are 59% of the way to the following notes.

When the first and third notes are given twice as much length as the second and fourth notes, the rhythm assumes a triplet feel. It can actually be conceived as a triplet based rhythm as in the box below. An example is provided below with the tamborine playing triplets in the background.

conga pattern, triplets

Here the displaced notes are 66.666%, 2/3 of the way to the following notes, resulting in a triplet feel.

The amount of swing (displacement) has a big effect on the feel of a rhythm, and different amounts of swing feel different at different tempos.

To illustrate how this works, we've taken a generic rhythm, call it Rhythm X, or RX.MID, which bears a slight resemblance to the W. African rhythm "Lamban", at three different tempos, with three varying degrees of swing; 50% or none at all, 59%, and 66.666%, or the triplet feel. Remember, these are only 3 choices from an infinite sliding scale. First at a moderately slow tempo:

RX.MID, slow, 50%, or no swing at all

RX.MID, slow, 59%, about halfway between straight and triplets

RX.MID, slow, 66.666.%, or triplet feel

Here are the same patterns at a bright tempo:

RX.MID, bright, 50% swing (none)

RX.MID, bright, 59% swing

RX.MID, bright, 66.666 swing

Finally, here they are fast:

RX.MID, fast, 50%
RX.MID, fast, 59%
RX.MID, fast, 66.666%

You can make the comparison here, using the standard jazz cymbal ride rhythm. Three different tempos, four different degrees of swing. Which do you prefer? Each is useful in the right situation.