By Burr is meant a thin protrusion of metal on an edge. Normally, a burr can only be felt from one side of the edge, but not from the other. However, it can be bent over to the other side by stropping. Under the magnifying glass, it looks like a thin wire along the edge. It is removed by repeated grinding/folding from side to side. Around the topic some very special legends entwine (see e. g. here).
Let me summarize some details as far as I understood them:
when grinding a bevel correctly, a burr is always created (except for the Belgian whetstone). With coarse stones it can be felt well with the finger, with finer stones less well. You can also feel it with your fingernail.
if at the beginning of the work with a certain grit you did not have a burr, the existence of a continuous burr is an indication that the grinding process with this grit is now complete.
the last point shows a possible way of working: Each grit is processed with 2 steps:
Grinding until a burr is formed
Removing the burr with the same grit
usually the burr can be felt from one side, but not from the other.
it can be difficult to grind off the burr from the tactile side. It may bend back, which can lead to additional steps. The temptation is great to raise the corresponding area a little. This could distort the cutting edge angle or ruin the cutting edge.
Normally, the last burr is removed with a stropping leather. But be careful: really only pull the iron, otherwise you quickly damage the leather.