A small capacitor placed right next to a chip's power pin to steady its supply; distance matters, so placement rules exist for it.
How it looks
Real image: CORE3588, rendered by KiCad from our board file
Real render, right edge of CORE3588 (8 x 7.7 mm). Two identical chips, each with its own capacitor about 2 mm from its power pin.
1C45, a 100 nF decoupling capacitor.
2Pin 5 of U15 (a small buffer chip): its 3.3 V power pin. One short track joins it to C45.
3C44, the same job for U14 just above.
4U15 itself.
Real geometry: CORE3588 copper, drawn from our board file (2D plan view)
Same area, copper only, with the 3.3 V supply net in amber: the capacitor pad and the chip's power pin are one piece of copper, a short hop apart. The capacitor's other pad goes to ground.
1Capacitor pad on the 3.3 V net.
2Chip power pin on the same net.
How it works
Where it fits
The same part as the power-group entry decoupling cap; this entry is about where it goes. A larger, slower reservoir is a bulk cap.