Since a picture is worth a thousand words (maybe only 327 words after inflation) I'll show an example of 1891 Low Shield here. The difference is at the very bottom where the shield intercepts the date. Basically it comes down to how close the bottom tip of the shield gets to the rim, and how far apart the numerals in the date must be split apart. Here is a quick explanation I copied from a source for the difference and why it's so rare:
"The reverse shield was lowered at some point after 1887 to improve metal flow. 1890 dated coins have a majority of the new lower shield. As the old high shield dies wear out and are replaced with the new lower shield dies, coins with the high shield become progressively rarer. According to David Iverson and Steve Hill in their publication ‘The Jubilee Gold Half Sovereign 1887-1893’ (Sovereign Rarities, 2019), all coins struck in 1891 are the lower shield type. In 1892, with production at 13.7 million coins, a small number of high shield coins exist, so old dies from 1887 production runs were used in 1892 to cover production gaps when necessary. However, with the relatively low mintage of 1891 the use of old dies seems unnecessary."
One of the main reasons for the High Shield was to allow for the mintmark to be placed under it for coins minted in Australia. So all Australian Victoria Jubilee Head half-sovereigns are necessarily of the High Shield variety. I've also shown 1891-S (Sydney mint) High Shield as example. No mintmark appears on coins minted in London, and so after 1887 the vast majority are of the Low Shield variety.