1)
What is "Shtei Lechem"?
1.
Radak citing Targum Yonasan: It is two Chalos. Yonason translated "Sheloshes Kikaros" (verse 3) to be Pitim, which are larger loaves. He was carrying also the Chalos; the verse mentioned only the large loaves. If he was carrying only the three, he would not give two of them to Sha'ul and his youth, and leave only one for the three of them! Also the Torah distinguished - "v'Kikar Lechem Achas v'Chalas Lechem Achas" (Shemos 29:23).
2)
Why should Sha'ul accept from them?
1.
Malbim: They will support you to protect your Shalom, and fill your physical needs.
