Bull means yields are falling and bond prices rising. Steepener means the long-short gap is widening. Put together: the 2-year is collapsing on expectations of rate-cuts while the 10-year drifts down more slowly.
This is the shape you see when the labour market cracks or a credit scare hits. It is often the final stage before or during a recession, and it usually coincides with front-end volatility and a weaker dxy.
Example: the 2-year falls from 4.60% to 3.90% while the 10-year falls from 4.20% to 4.05%. Both rallied, but 2s10s went from -40 bp to +15 bp, a 55 bp bull steepening.
Related: bear-steepener, bull-flattener, curve-steepener, twos-tens