A Sudoku is a grid of 9 × 9 cells, with some numbers already filled in. The goal is to fill in the rest so that every row, every column and every box has the numbers 1 to 9, each exactly once.
That is the only rule. Everything else (the techniques these articles teach) is a way of drawing conclusions from it without guessing. A well-made puzzle has a single solution and can be solved by logic alone. Every Urso Pardo puzzle is like that.
The cell in row 6, column 5 belongs to three units at once: row 6, column 5 and box 5.
Two cells in the same unit see each other and cannot hold the same number. Every cell sees 20 others: 8 in its row, 8 in its column and 4 more in its box that are in neither of the two.
The candidates of an empty cell are the numbers that can still go there: those that appear in none of the 20 cells it sees. When a cell is left with a single candidate, that is its number: it is a naked single.
The row, the column and the box of the cell in row 6, column 5 already hold the 2, 3, 5, 7 and 9. That leaves four candidates: 1, 4, 6 and 8.
Writing the candidates in the cells is called taking notes, a subject with its own article. You don't need it to begin with: the first techniques can be seen just by looking at the grid.
A technique is a rule that, in a specific pattern, places a number or removes candidates. Difficulty is measured by techniques: an Easy puzzle can be solved with the Easy techniques alone; a Hard one needs at least one of the hard ones.
A method is the way you search: where to look, how to take notes, in what order to try the techniques. The articles follow the order in which it is worth learning them, starting with scanning, the most natural way to find numbers in a grid full of gaps.