What does Pt mean in this formula?

[Meerio]

So I'm trying to understand something about the Hulse Taylor Binary. Here they talk about the change in periode that actually changes with the period. So what does P_t mean here?
And what is actually being explained? P_1year also makes no sense to me. Is that the period after one year? And what is deltaP_1year?
I'm sorry but I'm quite confused about the formula.
Thx for any help

http://astronomy.stackexchange.com/...is-cumulative-periastron-time-shift-calculate

 

[mfb]

P_t is the period as function of time, later evaluated at t=1 year and t=10 years.

$\Delta$ P_1year is the change in orbital period after 1 year relative to the original period. As you can see, the change in period over 10 years is nearly 10 times the change of period over 1 year, indicating a linear change of period over time - within the current timeframe and experimental precision.