What do 60i(50i) and 60p(50p) in the recording format AVCHD 60i/60p(50i/50p) stand for?

For NTSC region:

AVCHD 60i is the process of recording 60 frames per second using the interlace scanning system while 60p records 60 frames per second using the progressive scanning system. When AVCHD 28M (PS) is set, movies are recorded in 60p. Otherwise, they are recorded in 60i.

The interlace scanning system displays even-numbered and odd-numbered scanning lines alternately. The progressive scanning system displays even-numbered and odd-numbered scanning lines on the entire video frame at the same time, thus it has double the information per unit time compared with the interlace scanning system.

For PAL region:

AVCHD 50i is the process of recording 50 frames per second using the interlace scanning system while 50p records 50 frames per second using the progressive scanning system. When AVCHD 28M (PS) is set, movies are recorded in 50p. Otherwise, they are recorded in 50i.

The interlace scanning system displays even-numbered and odd-numbered scanning lines alternately. The progressive scanning system displays even-numbered and odd-numbered scanning lines on the entire video frame at the same time, thus it has double the information per unit time compared with the interlace scanning system.

[1] Interlace Scanning
[2] Progressive Scanning