Much software on the 8-bit Atari is highly interactive and timing-critical.
For example, many games are highly responsive to input from the game player
via a game controller (such as a joystick). The Atari software programmer
can reasonably assume that the video display device can reflect changes as
output by the Atari with no perceivable additional delay. Software timing
considerations are normally limited to the capabilities and constraints of
the Atari hardware itself.
However, in some cases a television or video monitor may introduce a
noticeable lag time between the video signal as input by the Atari, and the
video signal as actually displayed by the device. While any such effect is
truly negligible with CRT televisions or monitors (the standard of the time of
the Atari), the modern high-definition television (HDTV) has emerged as an
important exception. An HDTV typically dedicates considerable processing
efforts (hardware+software) toward presenting the most attractive picture
possible. Since few HDTV applications require precise interactive timing,
this video processing is generally not optimized for time efficiency. When
used with an older but timing-precise device such as an 8-bit Atari computer,
the resulting delay can be quite noticeable, making the HDTV essentially
unusable for "serious" action gaming.
Fortunately, many HDTVs offer a "game mode" that circumvents much of the video
processing otherwise performed, thereby eliminating most of the display lag
otherwise introduced by the television. If an HDTV is to be used with the
Atari for gaming, a "game mode" on the HDTV is essential.
Wikipedia on the topic: http://en.wikipedia.org/wiki/Display_lag