Capex means capital expenditure: the big money a company spends on long-lived assets like buildings, servers, and chips. When a hyperscaler (a giant cloud operator such as Amazon, Microsoft, or Google) builds a data center, that is capex.
Why watch it? Hyperscaler capex is growing +60%, and roughly 70% of DRAM now goes into data centers. Their spending plans are a direct signal of memory demand for the year ahead. Follow the capex, and you see the shortage coming.