In brief, BU:GU is a balance number where you divide the IBUs by the gravity points to the right of the decimal, e.g., take a beer with an original gravity of 1.060, knock off the 1.0, and you have 60 gravity points or GU. If the same beer also has 60 IBUs, then the BU:GU ratio is 60:60 = 1.00.
Now if a similar 1.060 beer had only 30 IBUs, then the ratio is 30:60 = 0.50.
In general, BU:GU values of about 0.50 are considered "balanced" with respect to bitterness vs. malt. Most IPAs, on the other hand, will have values >1.00, to a practical maximum of around 1.15-1.20 (above that and it's turning into hop soup a la NEIPA). And then on the other-other hand, a malty beer such as a Scottish ale or other low hopped beers like hefeweizen may have a lower ratio of just 0.25-0.30.
This ratio can be very useful for recipe design. If you know you want a beer that is a little more towards the malty side, and you want it to be about 6% alcohol, you may wish to set your BU:GU ratio a little lower than 0.50, maybe at 0.40, and set the original gravity at 1.060 (another shortcut rule is that you'll get roughly 1% ABV per 10 BUs). With this knowledge, you know you want BU:GU = 0.40 = X/60. Solving for X, you get 24. So you should aim for about 24 IBUs to get the amount of balance you want.
That's how I use BU:GU. And there are tables online that give the ratio for each well known style, to give you an idea of how well balanced they are between hops and malt.
Hope this helps. Cheers.