Some of the following material will assume an understanding of what this is, so here's background in case you don't. Simply put - it's the "official" position that party takes on political issues in a specific political cycle. The 2014 Texas Republican Party Platform - for example - is the "official" position the party has taken on issues topical prior to the 2014 election. The Texas Democrats will do the same when the meet.
Each party did so in each of the previous elections going back to the dawn of conventions - it's one of the reasons why they are held, and one of the reasons activists bother to get involved. It allows them to have influence on the policy making process. Each convention's party platform tends to be based on the previous one, but there are changes that can occur from time to time, and these reflect the changes in the party's power structure from election cycle to election cycle.
Not that I put the word official in quotes. This is because despite the fact that the platform reflects what the party stands for, it is largely the product of the passionate activists who dominate the state's temporary party organization.
It is not necessarily what the permanent party organization, or individual candidates stand for.
There is no legal mechanism for forcing candidates to run on the platform - though it may be politically problematic if they don't.
As we go forward, keep in mind that the platform is the product of internal political conflict within each party. Despite the fact that we have a tendency to think of parties as being monolithic entities, they have internal differences. These fights often play out over the tome and substance of the party platform.
Some - hopefully - helpful readings on the subject of party platforms.
- Wikipedia: Party Platform
- American Presidency Project: Party Platforms.
