The USCF is The United States Chess Federation. Membership in the USCF allows the chess player to participate in our National Rating System. A USCF membership is necessary to play in most tournaments in the US.
Players in 3rd grade or lower who are playing in a section that is restricted to 3rd grade and under (usually called K-3 or Primary) may play in that section as a non-member ("the JTP program"). The registrar can obtain a USCF ID for the player as a courtesy.  That ID number must be renewed to full membership as soon as the player reaches 4th grade or the player wishes to play in a section that includes players in 4th grade or higher.
Once you have a USCF ID, it is yours for life.  A membership can be renewed at any point, even after lapsing years previously.  The same goes for a rating.  It stays with the membership, going up and down according to the results of tournament play over the course of a person's lifetime.