Curse of Dimensionality
BoW produces one feature per vocabulary word, i.e. high dimensionality (listed as a con of BoW, s.71). The lecture names the “curse of high dimensionality” as the reason for Concept - Dictionary Reduction (02 Pre-processing s.73). The slide illustrates it with cubes: 5 cubes on one axis (X), a 5x5 grid in 2-D (X, Y), and a 5x5x5 block in 3-D (X, Y, Z): the amount of space grows rapidly with each added dimension.
Added context
Consequence (not stated on the slides): with many dimensions data becomes sparse and learners have difficulty (cf. 01 Intro s.30: “learners might have difficulty with high-dimensional data”).
Related
Concept - Feature Selection | Concept - Bag of Words | Concept - Sparse Matrix (CSR)