Question: I have a linear regression model for a dependent variable $Y$ based on two independent variables, $X1$ and $X2$, so I have a general form of a regression equation 

$Y = A + B\_1 \cdot X\_1 + B\_2 \cdot X\_2 + \epsilon$, 

where $A$ is the intercept, $\epsilon$ is the error term, and $B\_1$ and $B\_2$ are the respective coefficients of $X\_1$ and $X\_2$. I perform a multiple regression with software (statsmodel in Python) and I get coefficients for the model: $A = a, B\_1 = b\_1, B\_2 = b\_2$. The model also gives me $p$ values for each coefficient: $p\_a$, $p\_1$, and $p\_2$. My question is: What is the null hypothesis for those individual $p$ values? For example, to obtain $p\_1$ I know that the null hypothesis entails a 0 coefficient for $B\_1$, but what about the other variables? In other words, If the null hypothesis is $Y = A + 0 \cdot X\_1 + B\_2 \cdot X\_2$, what are the values of $A$ and $B\_2$ for the null hypothesis from which the $p$-value for $B\_1$ is derived?

Answer: 
The null hypothesis is 
$$
H\_0: B1 = 0 \: \text{and} \: B2 \in \mathbb{R} \: \text{and} \: A \in \mathbb{R}, 
$$
which basically means that the null hypothesis does not restrict B2 and A.
The alternative hypothesis is 
$$
H\_1: B1 \neq 0 \: \text{and} \: B2 \in \mathbb{R} \: \text{and} \: A \in \mathbb{R}.
$$
In a way, the null hypothesis in the multiple regression model is a composite hypothesis. It is "fortunate" that we can construct a pivotal test statistic that does not depend on the true value of B2 and A, so that we do not suffer a penalty from testing a composite null hypothesis.

In other words, there are a lot of different distributions of $(Y, X1, X2)$ that are compatible with the null hypothesis $H\_0$. However, all of these distributions lead to the same behavior of the the test statistic that is used to test $H\_0$. 

In my answer, I have not addressed the distribution of $\epsilon$ and implicitly assumed that it is an independent centered normal random variable. If we only assume something like 
$$
E[\epsilon \mid X1, X2] = 0
$$
then a similar conclusion holds asymptotically (under regularity assumptions).
You can make the same assumptions for the other variables as the X1. The ANOVA table of the regression gives specific information about each variable significance and the overall significance as well. As far as regression analysis is concerned, the acceptance of null hypothesis implies that the coefficient of the variable is zero, given a certain level of significance.

If you want to acquire a more intuitive aspect of the issue, you can study more about Hypothesis testing.