What does the letter U mean in movie rating?

there is U and PG and R what does they mean?

U stands for UNIVERSAL ( can be viewed by all age groups)
PG stands for PARENTAL GUIDANCE

MPAA's age-based ratings numerically as 1, 2, 3, and 4 for films rated G, PG, PG-13, and R, respectively (ie, we coded an R-rated film with the age-based rating of 4). Since the MPAA rating reasons do not follow any standard format, we coded each of the reasons with the classifications listed in Table 1 . The classifications include both the type of content (as a capital letter) and any modifying adjective for that content (as a lowercase letter). For example, we coded "violence" as "V," "action violence" as "aV," "strong violence" as "sV," and "sexual violence" as "zV." For some of our analyses, we further reduced the number of categories by combining content of similar natures; "violence" includes any rating reasons coded as Violence (V), Murder (M), Wrestling (W), Fighting (F), Rape (R), and Peril (K); "sex" includes any rating reasons coded as Sex (S), Sensuality (Q), Sexuality (J), Rape (R) (counted as violence and sex), Nudity (N); "substances" includes any rating reason coded as Drug (D), Alcohol (A), or Tobacco (T); and "thematic elements" includes any rating reason coded as Elements (E), Suicide (C), or Other (O). We also identified the animated films by searching their descriptions for indications of animation.

Unrated?

"U" stands for unrestricted. The complete break down for the US is:

Unrestricted
G - General Audiences - All ages admitted
PG - Parental Guidance Suggested - Some material may not be suitable for children.
PG-13 - Parents Strongly Cautioned - Some material may be inappropriate for children under 13.

Restricted
R - Restricted - Under 17 requires accompanying parent or adult 17 or older with photo ID
NC-17 - No one 17 and under is admitted -

Others
NR or Not Rated
M - For mature audiences (used 1969-71).
GP or General audiences鈥攑arental guidance suggested - In 1970-71
SMA - "Suggested for mature audiences".
X - The precursor to the current NC-17 rating

Unrestricted, Universal or Unrated (now you are really confused aren't you)!
PG is Parental Guidance and R is restricted (R18 is dirty mac time lol)

U- Universal
PG- Parental Guidance
and
R- Restricted.