Can anyone tell me what does this mean?

CREATE TABLE Members (
      MemberID int PRIMARY KEY ,
      FirstName varchar (25) NULL ,      -------->  what is the value (25) means and what's the NULL mean?? Confused.
 

Lets go line by line:

The first line simply creates a table and names it "Memembers". Then, declares a field/column names "MemberID" with a datatype of int(integer). Also, this field/column is the primary key of the table.

For 3rd line, you declare a fireld/column with a datatype "varchar". According to the documentation of MYSQL, Varchar datatype holds alphanumeric data (letters and numbers).  In this specific case, varchar(25) means that the column will take alphanumeric data of max length of 25.

Then, NULL means that the column will accept NULL values. Think of the "Webaddress" field in the the artists table in the lyric database(it has one NULL value for a webaddress). Instead, if you had the following: Firstname varchar(25) NOT NULL, then you would not be able insert NULL values in the table after creating the table.

Hope this helps.