רַק֩ חֲזַ֨ק וֶֽאֱמַ֜ץ מְאֹ֗ד לִשְׁמֹ֤ר לַעֲשׂוֹת֙ כְּכָל־הַתּוֹרָ֗ה אֲשֶׁ֤ר צִוְּךָ֙ מֹשֶׁ֣ה עַבְדִּ֔י אַל־תָּס֥וּר מִמֶּ֖נּוּ יָמִ֣ין וּשְׂמֹ֑אול לְמַ֣עַן תַּשְׂכִּ֔יל בְּכֹ֖ל אֲשֶׁ֥ר תֵּלֵֽךְ׃
Only be strong and very courageous, to observe to do according to all the law, which Moses My servant commanded thee; turn not from it to the right hand or to the left, that thou mayest have good success whithersoever thou goest.
Rashi on Joshua
Just be very strong and courageous. This was said in reference to the Torah, as He states: ‘To preserve, to act in accordance with all the laws of the Torah.’
Rashi on Joshua
תַּשְּׁכִּיל—You will be successful.13People consider that a successful person is a smart person. That is why the word תַּשְׂכִּיל—is associated with the word תַּצְלִיחַ. (Metsudas Tzion).