OK, usually you'd formulate it as in the second formula,
$$\{ \psi_{ru}\,\beta(d_{ru}) \ge \rho: r\in \mathcal B\}$$
would more succinctly be written as
$$\{ r | \psi_{ru}\,\beta(d_{ru}) \ge \rho\}$$
This is the set of DUs that meet the following criterion:
The product of path "loss" (more a "gain") (which depends on the distance from the user to the DU) and the shadowing (which is random drawn from a lognormal distribution) must be at least some threshold $\rho$. In other words, the signal power loss between DU and user must not be worse than $\rho$.
I’m not very strong in math
You're doing well! You're operating in a very math-heavy domain, and catching up on basics of notation is something best done at the beginning of studying of a domain. So, asking about things when you're confused is a good way forward.
As a recommendation: when you meet a term like that, make sure you first identify all syntax elements you know to make sure you are clear whether you generally understand the statement. Then, identify whether you can map the used variable symbols to meanings as defined in the text you're reading.
This two step process,
- make 100% sure you understand the syntax of a formula,
- "hunt down" the symbols used in the formula
is what I use all the time when facing new formulas in literature. The cognitive step of just understanding why you don't understand a formula is critical, and usually allows one to figure out the meaning of the syntax yourself.
