Shiddah means that the consonant's sound is fully stopped for a moment at its articulation point, followed by its proper release. It does not authorize artificially extending the consonant or adding a vowel.
Its eight letters are remembered through أَجِدْكَ قَطَبْتَ: ء، ج، د، ق، ط، ب، ك، ت.
Practice: feel the closure when pronouncing «أَقْ» and «أَكْ». Contrast it with unvowelled س, whose sound is able to continue. Qalqalah is a separate quality to be studied later; it does not follow that every shiddah letter receives qalqalah.