Having found this list online, could I ask for an explaination?

[Harry Hayfield]

State Filing Deadline Primary Runoff (if applicable)
Alabama  April 2 June 1 July 13
Alaska  June 1 Aug. 24   
Arizona  May 26 Aug. 24   
Arkansas  March 8 May 18 June 8
California  March 15 June 8   
Colorado  May 29 Aug. 10   
Connecticut  May 25 Aug. 10   
Delaware  July 30 Sept. 14   
District of Columbia July 7 Sept. 14   
Florida  April 30 Aug. 24   
Georgia  April 30 July 20 Aug. 10
Hawaii  July 20 Sept. 18   
Idaho  March 19 May 25   
Illinois  Nov. 2 Feb. 2   
Indiana  Feb. 19 May 4   
Iowa  March 19 June 8   
Kansas  June 10 Aug. 3   
Kentucky  Jan. 26 May 18   
Louisiana  July 9 Aug. 28 Oct. 2
Maine  March 15 June 8   
Maryland  July 6 Sept. 14   
Massachusetts  May 4 Sept. 14   
Michigan  May 11 Aug. 3   
Minnesota  July 20 Sept. 14**   
Mississippi  March 1 June 1 June 22
Missouri  March 30 Aug. 3   
Montana  March 15 June 8   
Nebraska  Feb. 15* May 11   
Nevada  March 12 June 8   
New Hampshire  June 11 Sept. 14   
New Jersey  April 12 June 8   
New Mexico  Feb. 9 June 1   
New York  N/A Sept. 14   
North Carolina  Feb. 26 May 4 June 22
North Dakota  April 9 June 8**   
Ohio  Feb. 18 May 4   
Oklahoma  June 9 July 27 Aug. 24
Oregon  March 9 May 18   
Pennsylvania  March 9 May 18   
Rhode Island  June 30 Sept. 14   
South Carolina  March 30 June 8 June 22
South Dakota  March 30 June 8 June 29
Tennessee  April 1 Aug. 5   
Texas  Jan. 4 March 2 April 13
Utah  March 20 June 22**   
Vermont  July 19 Sept. 14   
Virginia  April 9 June 8***   
Washington  June 11 Aug. 17   
West Virginia  Jan. 30 May 11   
Wisconsin  July 13 Sept. 14   
Wyoming  May 28 Aug. 17   


*Feb. 15 is the filing deadline for incumbents and March 1 is the filing deadline for non-incumbents; **Earlier nominating conventions; ***Parties can choose between a primary or holding a nominating convention. Source: State boards of election

(i.e What is a filing deadline? Are the primaries all for one party (i.e GOP voting for GOP candidates, Dem voting for Dem candidates, Ind not allowed to vote except for states where they can)?

[JohnnyLongtorso]

The filing deadline is the date at which candidates' nominating petitions and filing fees have to be remitted. Generally, major party candidates have an earlier deadline than independent and minor party candidates, because the latter don't have primaries. The ones listed here are for major party candidates.

Depending on the state, primaries can be closed to voters registered with a particular party, can be open to independent voters as well, or can be open to all voters (typically in states with no party registration, like Virginia and Montana).