Technical notation is a set of abbreviations that indicate what techniques are present in a chart. Most modern packs in Tech and FA feature tech notation. It is entered in the Simfile Author field in .sm files or Description field in .ssc files. To see tech notation, you need to use a theme which displays it, such as Simply Love.
This video gives a good overview of technical notation.
DS+ XO BR-
This chart has a lot of double steps (DS), some crossovers (XO), and a few brackets (BR).
+ and - - Plus after an abbreviation means the chart has a lot of that tech. Minus means there isn't much.
BR - Brackets - Two arrows intended to be hit with one foot. The player puts their foot diagonally across two panels, such as left and up.
BU - Bursts - Fast, short sequences of notes, usually 24ths, 32nds, or 16ths at high BPMs.
DS - Double Steps - Steps where a player hits 2 consecutive notes, on different arrows, with the same foot. DS are often marked with a mine under the first step, or a hold during the steps.
FS - Footswitches - Two or more steps on the same arrow (either up or down) where the player alternates feet. These are often marked with a mine.
GH - Ghoststeps - Intended/Forced ghost steps. Stepping on a note that isn't actually there, usually used to keep rhythm in less dense sections.
JA - Jacks - Two or more steps on the same arrow that are hit with the same foot.
KS or DSFS - Kickswitches/Doublestep-Footswitches - A combination of a footswitch and a doublestep in which the player is forced to doublestep onto the panel their other foot is resting on. These are typically marked with a hold and a mine.
MD or MA - Mine Dodge/Avoidance - Mines must be actively dodged by the player, as opposed to them being used purely to signal other forms of tech. Don't confuse MA here with Marvelous Attack in DDR.
RH or SK - Rhythms (sometimes called Skittles) - Unusual or difficult rhythms such as tuplets and odd time signatures.
SS - Sideswitches - Two or more steps on one of the side panels (right or left) where the player alternates feet. A player usually has to crossover into or out of a sideswitch. These are often marked with a mine.
HS or WA - Holdswitches (Sometimes called Wadats or Wadatsumis) - A long hold with alternating arrows requiring the player to temporarily release the hold in order to hit the pattern, and then step back on the hold with their other foot.
STR - Stream - Continuous streams of arrows, usually 16ths. May be followed by Breakdown Notation which is used in Stamina_/_Footspeed charts, although this is done less now that Simply Love calculates stream breakdowns automatically.
XMOD - X Speed Mod - Song contains Gimmicks such as speed changes and stops. In some tournaments, the player may not use C-Mod on these files.
XO - Crossovers - Steps where a player "crosses over" and hits the left arrow with their right foot or vice versa.