What you learned in school
Biology and evolution textbooks taught Lamarck's theory as accepted scientific fact: that organisms could pass on traits acquired during their lifetime to their offspring. Students learned that if a giraffe stretched its neck to reach leaves, its offspring would inherit longer necks. Blacksmiths' children would have stronger arms. This "use and disuse" principle seemed logical and was widely taught as the mechanism of evolution before genetics was understood.
What we know now
Before genetics was fully understood, Lamarck's theory of inheritance of acquired characteristics was taught as scientific fact. Students learned that if a giraffe stretched its neck to reach high leaves, its offspring would have longer necks. Blacksmiths's children would have stronger arms. This seemed logical and was widely accepted. Darwin's theory and later genetic research proved that acquired traits during an organism's lifetime are not passed to offspring through DNA. However, modern epigenetics has shown some environmental factors can affect gene expression across generations, adding nuance to this old debate.